According to the US 2011 Wiretap Report, encryption — on the off chance that it is encountered — has been no hurdle to retrieving the content of a conversation.

Public Law 106-197 amended 18 U.S.C. § 2519(2)(b) in 2001 to require
that reporting should reflect the number of wiretap applications
granted in which encryption was encountered and whether such
encryption prevented law enforcement officials from obtaining the
plain text of the communications intercepted pursuant to the court
orders. In 2011, encryption was reported during 12 state wiretaps, but
did not prevent officials from obtaining the plain text of the
communications.

— US 2011 Wiretap Report, page 8–9 (emphasis added)

Why was this encryption not successful?

Unfortunately the report includes very little detail about what kind of transmission and encryption was involved in any of the 12 cases.

You can add "email stored on the servers of the email providers such a gmail". So while the communication with gmail is encrypted using SSL, the stored mails are not.
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Hendrik Brummermann♦Jun 30 '12 at 12:40